This invention relates to a seal system for rotary machines and in particular to a seal system which maintains a positive seal for process gas conditions that may fluctuate from a vacuum to a substantial positive pressure.
Sealing systems employing "liquid-to-gas" seals to prevent high pressure working substances contained within a rotary machine, such as a turbine or compressor, from escaping about the shaft to regions of lower pressure or to the atmosphere are quite extensively commercially employed. The term "liquid-to-gas" seal, as used herein, generally refers to a sealing system utilizing a pressurized substance which acts as a dam against the working substances to prevent the working substance from passing into a region of lower pressure. It should be understood that the term "liquid-to-gas" is broad enough to include any fluid-to-fluid arrangement capable of producing the desired results.
In some process gas applications, it is essential that the seal fluid be isolated from the process gas. To achieve the desired total segregation, a small volume of process gas is injected between the process gas and the seal fluid to establish a buffer or barrier. The buffer gas is at a slightly higher pressure than the seal fluid, insuring that the seal fluid will not contaminate the process gas.
In some applications, the process gas inlet pressure may vary from a vacuum to a relatively substantially positive pressure. Generally, the buffer gas is injected at a relatively high pressure region of the rotary machine. From the injection point, the buffer gas flows axially in opposite directions along the shaft of the machine. In one direction of flow, the gas establishes the barrier to the seal fluid, whereas in the other direction of flow, the buffer gas is generally returned to the inlet or other lower pressure region of the machine for mixture with the process gas. Generally, labyrinth or similar flow restricting seals are interposed between the injection region and the inlet and seal fluid barrier regions to limit the quantity of buffer gas required to establish the desired fluid barrier.
However, in applications wherein the inlet pressure may vary from a vacuum to a relatively substantially positive pressure, the typical buffer gas arrangement of the type described has not proven totally satisfactory.
In arrangements of the prior art, if the quantity of buffer gas supplied to the barrier region is established when the inlet pressure is under vacuum conditions, then an excessive quantity of buffer gas will be supplied to the barrier region when the inlet pressure reaches a relatively substantial positive pressure. In addition, as the inlet gas pressure increases, there exists a possibility that lower pressure at the barrier region will result in the flow of process gas toward the seal fluid injection region thereby causing possible problems, such as solidification of the seal fluid. Conversely, if the quantity of buffer gas delivered to the injection region is determined when the inlet pressure is at a relatively substantial positive pressure, then there exists a strong probability that an insufficient quantity of buffer gas will flow to the barrier region when the inlet pressure is reduced, thereby permitting the seal fluid to mix with and perhaps contaminate the process gas.
Essentially, the variations in flow to the barrier region are caused by the variations in the inlet pressure of the process gas. As inlet pressure decreases, and in some instances approaches or becomes a vacuum, there is an increase in the flow of buffer gas from the injection region to such low pressure region due to the substantial pressure differential therebetween. If the supply of buffer gas to the injection region is determined when the process gas inlet pressure is at a relatively substantial positive pressure, and the flow of gas to the injection region remains constant, there will be insufficient flow of buffer gas to the barrier region when the inlet pressure decreases. Conversely, as the inlet pressure increases, the pressure differential between the injection region and the region functioning at inlet pressure decreases causing a concomitant reduction in flow of buffer gas to such region. If the supply of buffer gas to the injection region remains constant, there is a resultant increase in the flow of buffer gas to the barrier region. Since the buffer gas mixes with the sealing fluid in the barrier region, the gas forming the barrier becomes contaminated. An excessive flow of buffer gas to the barrier region thus reduces the efficiency of operation of the system utilizing the rotary machine.